Montana Governor and First Lady's Math and Science Initiative
Glacial Lake Missoula and Transportation Along the Clark Fork
I-90 Parking area two miles west of Alberton
During the last ice age about 15,000 years ago,
an enormous glacier pushed down from British
Columbia and blocked the Clark Fork River
in northern Idaho. The glacier functioned as an ice dam
creating the largest glacial lake known to have existed,
Glacial Lake Missoula. The lake’s waters backed up into
the river’s drainage in western Montana, creating a body
of water comparable to today’s Lake Ontario. As the
lake filled and water at the ice dam deepened, it caused
the lighter glacial ice to float and eventually break up,
triggering floods of epic proportions. The water flushed
through the Clark Fork drainage west of here enroute
to the Pacific Ocean. The torrent scarred the landscape
of eastern Washington, creating scablands that still
define the landscape. The geologic record indicates that
Glacial Lake Missoula filled and emptied on a cyclical
basis over a period of about two thousand years. Indeed,
the large road cut where the Interstate 90 bridge crosses
the Clark Fork River at Nine Mile, ten miles east of
here, preserves the record of at least 36 separate fillings
of the lake. Other evidence of the glacial floods include
ancient ice age shorelines on the mountains around
Missoula.
Glacial Lake Missoula Road Sign
A Brief History
In 1860, 150 men under the command of
Lieutenant John Mullan carved a wagon road
through the colorful Precambrian mudstones
on the mountainside north of here. The road
took six weeks to construct and required the use
of explosives to blast a route through the rocks.
Called the Point of Rocks Segment of the Mullan
Road, the road still traces its way across the
mountainside above here. In 1908, the Chicago,
Milwaukee, St. Paul & Pacific (Milwaukee
Road) Railroad constructed its transcontinental
line through the Clark Fork canyon enroute to
Seattle. The railroad also excavated tons of rock
to cut its way through these mountains to St.
Paul Pass. The old railroad grade, later known
as the Route of the Hiawatha for the celebrated
passenger train that once used the line, is still
evident along the north side of Interstate 90.
In 1914, the Yellowstone Trail,
blazed by distinctive chrome
yellow signs with black arrows,
passed through this canyon. The
trail became U.S. Highway 10 in
1926. Interstate 90 bypassed it here in 1963.
